The present invention relates to a system of control electrodes for an electrolytic display means with seven segments. It is used in the construction of display or indication members (watches, instrument panels, measuring apparatus, etc.).
An electrolytic display cell with seven segments comprises a counter-electrode, facing which there are seven transparent electrodes in the form of six peripheral electrodes and one central electrode. Each electrode is supplied with current by a current lead, itself connected to a connector permitting a connection with appropriate supply means. An electrolyte is placed between the electrodes and the counter-electrode and is able to modify the appearance of the electrodes, particularly by the deposition and then dissolving of a metal coating under the effect of the passage of current.
The control of electrolytic cells causes specific problems not encountered with other types of cells, e.g. with liquid crystals or electrochromic material. Thus, in the case of electrolytic display it is essential that there is a good uniformity of the potential applied to the excited electrodes, if not the display contrast obtained does not have the necessary uniformity. This is due to the electrochemical nature of the reaction occurring on the electrodes. This problem does not occur in the case of liquid crystal cells where the current is approximately 1,000 times lower than in electrolytic cells (a few .mu.A/cm.sup.2 instead of a few mA/cm.sup.2). In addition, said current is a stray current and not a controlled current. It also does not exist in the case of electrochromic devices (e.g. using WO.sub.3) for which the appearance or disappearance of colouring is less critical.
This is the reason why the known electrode systems for liquid crystal or electrochromic layer cells are unsatisfactory when used for electrolytic cells.
The problem of contrast uniformity in electrolytic cells is made more difficult by the fact that the current leads and electrodes are formed from thin conductive oxide coatings which, due to their thinness, have a by no means negligible resistivity.
This problem involves three specific difficulties:
(a) obtaining an equal voltage drop along the current leads between the connector and the electrodes; PA1 (b) ensuring a potential uniformity over the entire surface of each electrode; PA1 (c) solving the two above points for all the segments, including the central segment, whose access is difficult because it is surrounded by the six peripheral segments.
On using an apparatus of the type described, for example, in German Patent Application 2,731,718 entitled "Elektrochromes Anzeige-element", the central segment will be the seat of a potential gradient, because it is supplied by one of its small sides. Although this problem is not serious for an electrochromic material cell it is prohibitive with an electrolytic cell. On using an apparatus of the type described, for example, in U.S. Pat. No. 3,702,723 entitled "Segmented Master Character for Electronic Display Apparatus", the voltage drop is not the same over all the current leads between the connector and the electrodes and the segments will not all have the same appearance.